Method And Apparatus For Producing Carbon Monoxide By Cryogenic Distillation

ABSTRACT

A method and an apparatus for producing carbon dioxide by cryogenic distillation is provided.

The present invention relates to a method and an apparatus for producing carbon monoxide by cryogenic distillation. In known methods for producing hydrogen and/or carbon monoxide by cryogenic distillation, use is frequently made of a denitrogenation column in order to obtain products that are very low in nitrogen from a mixture to be separated that is contaminated with nitrogen.

It may however happen that the nitrogen content of the mixture to be separated is extremely low or even zero from time to time, while the unit has been designed with a denitrogenation column. This will thus be the case with a mixture to be separated produced by an SMR, fed temporarily with naphtha. Since regulation of the pressure of the denitrogenation column is made by controlling the nitrogen purge, it will no longer be possible to keep the pressure constant. Since this pressure controls that of the CO/CH₄ column generally upstream to the denitrogenation column, the operation of the unit will be caused to deteriorate considerably by probable fluctuations.

One means of avoiding these problems at relatively little cost consists of installing a new gas outlet on the CO/N₂ column, preferably in the vessel, and of connecting it to the low pressure carbon monoxide network. Thus, when the flow rate of the nitrogen purge becomes too low for regulating the pressure, it is sufficient to open the new carbon monoxide outlet and to regulate the pressure by this channel. The nitrogen purge should then be closed.

According to one object of the invention, a method is provided for producing carbon monoxide by cryogenic distillation using a system of distillation columns fed by a mixture comprising at least hydrogen, carbon monoxide and for at least part of the time nitrogen, in which the mixture to be separated is cooled to a distillation temperature, the mixture is purified from hydrogen, a fluid is separated comprising at least carbon monoxide and at least part of the time nitrogen in a denitrogenation column in order to produce a nitrogen-rich gas and a carbon monoxide-rich liquid and a carbon monoxide-rich gas is withdrawn from the system of columns, characterized in that if the nitrogen content of this fluid comprising at least carbon monoxide and at least part of the time nitrogen falls below a given threshold, a carbon monoxide-rich flow is withdrawn from the denitrogenation column and the pressure of the column is varied by regulating this carbon monoxide-rich flow.

According to another object of the invention, an apparatus is provided for the production of carbon monoxide by cryogenic distillation using a system of distillation columns fed by a mixture comprising at least hydrogen, carbon monoxide and at least part of the time nitrogen, comprising means for cooling the mixture to be separated to a distillation temperature, means for purifying the mixture in hydrogen, a denitrogenation column for separating a fluid comprising at least carbon monoxide and at least part of the time nitrogen, so as to produce a nitrogen-rich gas and a carbon monoxide-rich liquid, and means for withdrawing a carbon monoxide-rich gas from the system of columns, characterized in that it comprises means for withdrawing a carbon monoxide-rich gas from the denitrogenation column and means for varying the pressure of the denitrogenation column by regulating this carbon monoxide-rich flow.

Preferably, the denitrogenation column has a head condenser, means for conveying a liquid from the vessel of the denitrogenation column to the head condenser, means for withdrawing vaporized vessel liquid vessel from the head condenser and means for mixing the vaporized vessel liquid with the carbon monoxide-rich gas.

The invention will be described in greater detail with reference to the FIGURE that illustrates an apparatus for producing carbon monoxide according to the invention.

The apparatus comprises at least two distillation columns, comprising a CO/CH₄ column 5 and a denitrogenation column 11.

The apparatus may form part of an installation with a methane washing step or a partial condensation step upstream to the CO/CH₄ column in order to purify the mixture and separate the hydrogen from it. The denitrogenation column may also be situated upstream of the CO/CH₄ column.

The CO/CH₄ column is fed by two flows 1, 3 at different levels and produces a methane-rich liquid 7 and a fluid 9 carbon monoxide-rich fluid 9 but containing nitrogen, at least from time to time. The fluid 9 is conveyed to the denitrogenation column 11 in the upper part of the column. The vessel liquid 13 is conveyed to a head condenser 23 of the denitrogenation column 11 so as condense nitrogen 25. A purge flow 27 is withdrawn from the condenser 23 and conveyed to a heat exchanger 29 where it vaporizes and is expanded in a valve 31. The pressure of the denitrogenation column 11 is maintained by means of the purge flow 27.

If the nitrogen content of the flow 9 falls below a given threshold, the pressure of the column 11 can no longer be maintained with the purge flow. In this case, a flow of gaseous carbon monoxide 17 is withdrawn from the vessel of the denitrogenation column. By regulating the opening of the valve 19, the flow 17 may be varied and thus the pressure of the denitrogenation column 11 and therefore the pressure of the CO/CH₄ column. Carbon monoxide 17 is optionally mixed with the vaporized liquid 21 coming from the condenser 23.

The flows 13, 21 and 25 may be zero. 

1-3. (canceled)
 4. A method for producing carbon monoxide by cryogenic distillation using a system of distillation columns fed by a mixture comprising at least hydrogen, carbon monoxide and for at least part of the time nitrogen, the method comprising: cooling the mixture to be separated to a distillation temperature, purifying the mixture from hydrogen, separating a fluid comprising at least carbon monoxide and at least part of the time nitrogen in a denitrogenation column in order to produce a nitrogen-rich gas and withdrawing a carbon monoxide-rich liquid and a carbon monoxide-rich gas from the system of columns, wherein if the nitrogen content of the fluid comprising at least carbon monoxide and at least part of the time nitrogen falls below a given threshold, a carbon monoxide-rich flow is withdrawn from the denitrogenation column and the pressure of the column is varied by regulating this carbon monoxide-rich flow.
 5. An apparatus for producing carbon monoxide by cryogenic distillation using a system of columns fed by a mixture comprising at least hydrogen, carbon monoxide and for at least part of the time nitrogen, comprising means for cooling the mixture to be separated to a distillation temperature, means for purifying the mixture in hydrogen, a denitrogenation column for separating a fluid comprising at least carbon monoxide and for at least part of the time nitrogen, derived from the mixture, so as to produce a nitrogen-rich gas and a carbon monoxide-rich liquid and means for withdrawing a carbon monoxide-rich gas from the system of columns, further comprising a means for withdrawing a carbon monoxide-rich flow from the denitrogenation column and means for varying the pressure of the denitrogenation column by regulating this carbon monoxide-rich flow.
 6. The apparatus of claim 5, wherein the denitrogenation column comprises; a head condenser, means for conveying a vessel liquid of the denitrogenation column to the head condenser, means for withdrawing vaporized vessel liquid from the head condenser and means for mixing the vaporized vessel liquid with the carbon monoxide-rich flow. 